1. Field of the Invention
The present invention relates to a bismuth ferrite perovskite oxide and an oxide composition/oxide body/piezoelectric device/liquid discharge apparatus using the same.
2. Description of the Related Art
Perovskite oxides having ferroelectricity are used in various applications, such as piezoelectric devices, switching devices, and the like. For example, as a perovskite oxide having excellent piezoelectric properties, lead zirconate titanate (PZT) is known. PZT is a ferroelectric body having spontaneous polarization in the absence of electric field, and said to show high piezoelectricity at and near a morphotropic phase boundary (MPB). If environmental burden is considered, it is preferable that Pb content is reduced as much as possible and a non-lead system that does not include Pb is more preferable. In non-lead perovskite oxides, development of new materials having high piezoelectricity has been in progress.
As a non-lead perovskite oxide having high piezoelectricity (ferroelectricity) in theory, Bi perovskite oxides have been under study. Most of the Bi perovskite oxides, however, hardly, and some of them can not, take a perovskite structure by high-temperature sintering under normal pressure.
Currently, BiFeO3 is the only Bi perovskite oxide, in bulk ceramics, that can be manufactured into a Bi perovskite oxide under normal pressure. Thus, BiFeO3 can also be easily made into a Bi perovskite oxide in thin films. As having a high Curie temperature and a large spontaneous polarization, BiFeO3 has been studied as a prospective material of dielectric memories and piezoelectric actuators. But, BiFeO3 has a low dielectric constant and a high coercive electric field Ec in addition to leakage, so that a high piezoelectric constant can not be expected from BiFeO3 alone.
Consequently, a perovskite oxide film made of a material provided by solid solving BiFeO3 into barium titanate (BaTiO3), which is known as a non-lead perovskite oxide, is disclosed in U.S. Pat. No. 7,586,234 (Patent Document 1). The BiFeO3—BaTiO3 (BF—BT) perovskite oxide, however, has a problem that leakage current is likely to occur due to Fe valence change in BiFeO3.
In order to improve leakage characteristics, a technique for adding Mn in B-site of a BF—BT perovskite oxide is reported as described, for example, in U.S. Patent Application Publication No. 2009243438 (Patent Document 2). Patent Document 2 discloses a perovskite oxide film made of a material provided by adding and solid solving Mn to B-site of BiFeO3 and Zr to B-site of BaTiO3.
In the mean time, the BiFeO3—BaTiO3 (BF—BT) perovskite oxide has relatively high piezoelectricity among non-lead systems, but it has, for example, in a thin film state, a problem of low piezoelectricity under a low electric field of not greater than 30 V/1 μmt in comparison with a lead system. One of the factors for this would be a high Curie temperature of the BF—BT system. It is known that, as illustrated in FIG. 10, there is a correlation between the Curie temperature and piezoelectric d constant, and the BF—BT system has a Curie temperature of about 500° C. or higher. Consequently, attempts have been made to improve piezoelectric properties of the BF—BT system by decreasing the Curie temperature.
Patent Document 2 describes that a pinching effect is obtained by the addition of Zr. The “pinching effect” as used herein refers to the effect in which phase transition points of a compound, such as BaTiO3, having may phase transition points in a wide temperature range are collected to around a convenient temperature by solid solving, for example, BaSnO3 into BaTiO3 to reduce the variation with temperature in relative dielectric constant and to obtain a larger relative dielectric constant.
A literature by Rai et al., Material Chemistry and Physics, 119 (2010), pp 539-545 (Non-patent Document 1) discloses a Gd doped BiFeO3—BaTiO3 perovskite oxide in which the Curie temperature becomes 150° C. to 170° C.
For the Curie temperature of the BF—BT perovskite oxide, no phase transition temperature is observed from the liquid nitrogen temperature to 500° C., so that in Patent Document 2, it would be quite unlikely that the pinching effect in which phase transition temperatures of a crystal in a wide temperature range are collected to around a convenient temperature is applicable and the piezoelectric properties are rather degraded by the addition of Zr.
In the perovskite oxide of Non-patent Document 1, the Curie temperature is decreased to as low as 150 to 170° C. Although Non-patent Document 1 has succeeded in decreasing the Curie temperature, such Curie temperature would pose a problem of instability and unreliability when the perovskite oxide is used in a ferroelectric device, piezoelectric device, or a power generation apparatus, as the operation temperature will rise with time. For the application to such devices and apparatuses, for example, a Curie temperature not less than 200° C. and less than 500° C. is preferable.
The present invention has been developed in view of the circumstances described above and it is an object of the present invention to provide a novel non-lead perovskite oxide which is highly stable and has excellent ferroelectricity (piezoelectricity) under a low electric field.